When optical fibers are welded by means of the intense heat in an electric discharge the physical characteristics of the discharge change when the surrounding air pressure varies. Thus the electric voltage over the electrodes between which the discharge is formed is reduced when the air pressure decreases, which in turn results in a reduction of the electric current in the electric discharge and thus a lower temperature in the discharge region and thus in the region of the fibers heated by the discharge. It results in turn in inferior welded splices generally having a larger attenuation than splices welded at normal air pressure.
Thus, when welding optical fibers an electric discharge in air of atmospheric pressure is used, the discharge being self-igniting and self-sustaining and obtained when only applying a sufficiently high voltage between the cold, stationary welding electrodes, such a discharge being called a glow discharge, using the strict language of plasma physics. In the glow discharge electrons are emitted by secondary emission, principally owing to positive ions hitting the electrodes. When the current between the electrodes is increased from the values existing in the glow discharge, an arc discharge is obtained, and then the voltage between the electrodes is reduced to a very low value. A diagram illustrating the conditions when increasing from zero and on the voltage/current between two cold electrodes is shown in FIG. 1a. In region R1 a small voltage is applied over the electrodes and a very small current will then pass between the electrodes, which increases up to a saturation current. Region R2 is a transitional region reached when increasing the voltage even more and finally region R3 is reached which is the normal glow discharge region, producing a constant voltage between the electrodes called the burn voltage, the voltage in region R2 just before continuing to region R3 being called the ignition voltage. The glow voltage is determined by the exterior electrical circuit in which the electrodes are connected. When then increasing the current between the electrodes region R4 is obtained called abnormal glow discharge. Finally, in region R5 which is obtained when the current has been increased even more, an arc discharge is produced. In region R4 and even more in region R5 the high current density at the electrodes causes a heating thereof, finally resulting in that the negative electrode, the cathode, is so hot that electrons are emitted thermally and that the electrode voltage reaches a few or some ten of volts. The region R5 is used for ordinary welding of metals or similar materials, the initial heating of the negative electrode then being accomplished by heat generated in a contact resistance between the electrodes or between an electrode and the material to be welded, so that no high voltage is ever applied.
Different ways for compensating variations in the exterior air pressure when welding optical fibers using a glow discharge have been proposed:
The published European patent application EP A1 0 504 519 for Fujikura Ltd. discloses an optical fiber welding device comprising a discharge unit 2 for obtaining an electric discharge for melt-fusioning optical fibers, a pressure sensor 6 for providing a signal corresponding to the ambient atmospheric pressure and a "discharge control unit 3" for controlling the discharge in relation to the pressure signals so that to the discharge unit 2 an optimal current is delivered during the discharge. By using feedback through a line from a resistor 251 in the welding electrode circuit the welding current can be maintained constant. The Japanese patent application JP A 63-106706, which is available to the public, also for Fujikura Ltd. relates to an optical fiber welding device of a similar kind.
The German patent application DE A1 37 24 914 made available to the public, for kabelmetal electro GmbH, describes a procedure for connecting two optical fibers by means of the energy from an electric discharge. The procedure comprises among other things a control of the (gas) pressure which exists around the electric discharge and of the current through the electric discharge.
U.S. Pat. No. 5,002,351 for Szanto et al. relates to a melt welding device for optical fibers where it, according to the abstract, is arranged that "Once an arc is established between opposed electrodes, constant power is provided during the splicing." From the specification of this patent it is not apparent what is meant by "power" of how "constant power" can be achieved.